Flaviviruses are members of the genus Flavivirus, which is classified within the family Flaviviridae. The flaviviruses are largely pathogenic to humans and other mammals. Flaviviruses that inflict disease upon humans and animals include Alfuy, Apoi, Aroa, Bagaza, Banzi, Batu Cave, Bouboui, Bukalasa bat, Bussuquara, Cacipacore, Carey Island, Cowbone Ridge, Dakar bat, Dengue (serotypes 1, 2, 3 and 4), Edge Hill, Entebbe bat, Gadgets Gully, Iguape, Ilheus, Israel turkey meningoencephalitis, Japanese encephalitis, Jugra, Jutiapa, Kadam, Karshi, Kedougou, Kokobera, Koutango, Kunjin, Kyasanur Forest disease, Langat, Meaban, Modoc, Montana myotis leukoencephalitis, Murray Valley encephalitis, Naranjal, Negishi, Ntaya, Omsk hemorrhagic fever, Phnom Penh bat, Potiskum, Powassan, Rio Bravo, Rocio, Royal Farm, Russian spring summer encephalitis, Saboya, Sal Vieja, San Perlita, Saumarez Reef, Sepik, Sokuluk, Spondweni, St. Louis encephalitis, Stratford, Tick-borne encephalitis—central European subtype, Tick-borne encephalititis—far eastern subtype, Tembusu, THCAr, Tyuleniy, Uganda S, Usutu, West Nile, Yaounde, Yellow fever, Yokose, Ziki, Cell fusing agent and other related flaviviruses, as listed in Kuno et al. (J. Virol. 72: 73-83 (1998)).
The flaviviruses contain the following three structural proteins: prM/M, the premembrane and membrane protein; E, the envelope protein; and C, the capsid protein. (Monath, in Virology (Fields, ed.), Raven Press, New York, 1990, pp. 763-814; Heinz and Roehrig, in Immunochemistry of Viruses II: The Basis for Serodiagnosis and Vaccines (van Regenmortel and Neurath, eds.), Elsevier, Amsterdam, 1990, pp. 289-305). M has a molecular weight (MW) of about 7-8 kilodaltons (kDa) and E has a MW of about 55-60 kDa. M is synthesized as a larger precursor termed prM. The proportion of prM is removed when prM is processed to form M protein in mature virions. M and E are located in the membrane of the flavivirus particle, and so have long been considered to constitute important immunogenic components of the viruses.
The flaviviruses are RNA viruses comprising single stranded RNA having a length, among the various species, of about 10 kilobases (kb). The C protein, with a MW of 12-14 kDa, complexes with the RNA to form a nucleocapsid complex. Several nonstructural proteins are also encoded by the RNA genome which are termed NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5. The genome is translated within the host cell as a polyprotein, then processed co- or post-translationally into the individual gene products by viral- or host-specific proteases (FIG. 1).
The nucleotide sequences of the genomes of several flaviviruses are known, as summarized in U.S. Pat. No. 5,494,671. That for JEV is provided by Sumiyoshi et al. (Virology 161: 497-510 (1987)) and Hashimoto et al. (Virus Genes 1: 305-317 (1988)). The nucleotide sequences of the virulent strain SA-14 of JEV and the attenuated strain SA-14-14-2, used as a vaccine in the People's Republic of China, are compared in the work of Nitayaphan et al. (Virology 177: 541-552 (1990)).
Nucleotide sequences encoding the structural proteins of other flavivirus species are also known. In many cases, the sequences for the complete genomes have been reported. The sequences available include dengue serotype 1 virus, dengue serotype 2 virus (Deubel et al., Virology 155: 365-377 (1986); Gruenberg et al., J. Gen. Virol. 69: 1391-1398 (1988); Hahn et al. Virology 162: 167-180 (1988)), dengue serotype 3 virus (Osatomi et al., Virus Genes 2: 99-108 (1988)), dengue serotype 4 virus (Mackow et al., Virology 159: 217-228 (1987), Zhao et al., Virology 155: 77-88 (1986)), West Nile virus (Lanciotti et al., Science 286: 2331-2333 (1999)), Powassan virus (Mandl et al., Virology 194: 173-184 (1993)) and yellow fever virus (YFV) (Rice et al., Science 229: 726-733 (1985)).
Many flaviviruses, including St. Louis encephalitis virus (SLEV), WNV and JEV, are transmitted to humans and other host animals by mosquitoes. They therefore occur over widespread areas and their transmission is not easily interrupted or prevented.
West Nile fever is a mosquito-borne flaviviral infection that is transmitted to vertebrates primarily by various species of Culex mosquitoes. Like other members of the Japanese encephalitis (JE) antigenic complex of flaviviruses, including JE, SLE and Murray Valley encephalitis (MVE) viruses, WNV is maintained in a natural cycle between arthropod vectors and birds. The virus was first isolated from a febrile human in the West Nile district of Uganda in 1937 (Smithburn et al., Am. J. Trop. Med. Hyg. 20: 471-492 (1940)). It was soon recognized as one of the most widely distributed flaviviruses, with its geographic range including Africa, the Middle East, Western Asia, Europe and Australia (Hubalek et al., Emerg. Infect. Dis. 5: 643-50 (1999)). Clinically, West Nile fever in humans is a self-limited acute febrile illness accompanied by headache, myalgia, polyarthropathy, rash and lymphadenopathy (Monath and Tsai, in Clinical Virology, (Richman, Whitley and Hayden eds.), Churchill-Livingtone, New York, 1997, pp. 1133-1186). Acute hepatitis or pancreatis has been reported on occasion and cases of WNV infection in elderly patients are sometimes complicated by encephalitis or meningitis (Asnis et al., Clin. Infect. Dis. 30: 413-418 (2000)). Thus, infection by WNV is a serious health concern in many regions of the world.
The geographical spread of the disease, particularly the introduction of WNV into the U.S. in 1999, has greatly increased awareness of the human and animal health concerns of this disease. Between late August and early September 1999, New York City and surrounding areas experienced an outbreak of viral encephalitis, with 62 confirmed cases, resulting in seven deaths. Concurrent with this outbreak, local health officials observed increased mortality among birds (especially crows) and horses. The outbreak was subsequently shown to be caused by WNV, based on monoclonal antibody (Mab) mapping and detection of genomic sequences in human, avian and mosquito specimens (Anderson et al., Science 286: 2331-2333 (1999); Jia et al., Lancet 354: 1971-1972 (1999); Lanciotti et al., Science 286: 2333-2337 (1999)). Virus activity detected during the ensuing winter months indicated that the virus had established itself in North America (Morb. Mortal. Wkly. Rep. 49: 178-179 (2000); Asnis et al., Clin. Infect. Dis. 30: 413-418 (2000); Garmendia et al., J. Clin. Micro. 38: 3110-3111 (2000)). Surveillance data reported from the northeastern and mid-Atlantic states during the year 2000 confirmed an intensified epizootic/epidemic transmission and a geographic expansion of the virus with documentation of numerous cases of infection in birds, mosquitoes and horses, as well as cases in humans (Morb. Mortal. Wkly. Rep. 49: 820-822 (2000)).
Currently, no human or veterinary vaccine is available to prevent WNV infection and mosquito control is the only practical strategy to combat the spread of the disease.
Japanese encephalitis virus (JEV) infects adults and children and there is a high mortality rate among infants, children and the elderly in areas of tropical and subtropical Asia (Tsai et al., in Vaccines (Plotkin, ed.) W. B. Saunders, Philadelphia, Pa., 1999, pp. 672-710). Among survivors, there are serious neurological consequences, related to the symptoms of encephalitis, that persist after infection. In more developed countries of this region, such as Japan, the Republic of China (Taiwan) and Korea, JEV has been largely controlled by use of a vaccine of inactivated JEV. Nevertheless, it is still prevalent in other countries of the region.
Vaccines available for use against JEV infection include live virus inactivated by such methods as formalin treatment, as well as attenuated virus (Tsai et al., in Vaccines (Plotkin, ed.) W. B. Saunders, Philadelphia, Pa., 1994, pp. 671-713). Whole virus vaccines, although effective, do have certain problems and/or disadvantages. The viruses are cultivated in mouse brain or in cell culture using mammalian cells as the host. Such culture methods are cumbersome and expensive. Furthermore, there is the attendant risk of incorporating antigens from the host cells, i.e., the brain or other host, into the final vaccine product, potentially leading to unintended and undesired allergic responses in the vaccine recipients. There is also the risk of inadvertent infection among workers involved in vaccine production. Finally, there is the risk that the virus may not be fully or completely inactivated or attenuated and thus, the vaccine may actually cause disease.
Dengue fever and dengue hemorrhagic fever (DF/DHF) are caused by dengue virus, which is also a mosquito-borne flavivirus. There are four antigenically related, but distinct, dengue virus serotypes, (DEN-1, DEN-2, DEN-3 and DEN-4), all of which can cause DF/DHF. Symptoms of DF, the mild form of dengue-related disease, include fever, rash, severe headache and joint pain. Mortality among those subjects suffering from DF is low; however, among those subjects suffering from DHF, mortality can be as high as 5%. From available evidence, more than 3 million cases of DHF and 58,000 deaths have been attributed to DHF over the past 40 years, making DHF a major emerging disease (Halstead, in Dengue and Dengue Hemorrhagic Fever (Gubler and Kuno, eds.) CAB International, New York, N.Y., (1997) pp 23-44). Nevertheless, despite decades of effort, safe and effective vaccines to protect against dengue virus infection are not yet available.
Yellow fever is prevalent in tropical regions of South America and sub-Saharan Africa and is transmitted by mosquitos. Infection leads to fever, chills, severe headache and other pains, anorexia, nausea and vomiting, with the emergence of jaundice. A live virus vaccine, 17D, grown in infected chicken embryos, is considered safe and effective. Nevertheless, there remains a need for a vaccine that is stable under adverse conditions, such as are commonly encountered in the tropical regions of Africa and the Americas where the vaccine is most needed.
A recombinant flavivirus which is a chimera between two flaviviruses is disclosed in PCT publication WO 93/06214. The chimera is a construct fusing non-structural proteins from one “type,” or serotype, of dengue virus or a flavivirus, with structural proteins from a different “type,” or serotype, of dengue virus or other flavivirus. 
Several recombinant subunit and viral vaccines have been devised in recent years. U.S. Pat. No. 4,810,492 describes the production of the E glycoprotein of JEV for use as the antigen in a vaccine. The corresponding DNA is cloned into an expression system in order to express the antigen protein in a suitable host cell such as E. coli, yeast, or a higher organism cell culture. U.S. Pat. No. 5,229,293 discloses recombinant baculovirus harboring the gene for JEV E protein. The virus is used to infect insect cells in culture such that the E protein is produced and recovered for use as a vaccine.
U.S. Pat. No. 5,021,347 discloses a recombinant vaccinia virus genome into which the gene for JEV E protein has been incorporated. The live recombinant vaccinia virus is used as the vaccine to immunize against JEV. Recombinant vaccinia viruses and baculoviruses in which the viruses incorporate a gene for a C-terminal truncation of the E protein of dengue serotype 2, dengue serotype 4 and JEV are disclosed in U.S. Pat. No. 5,494,671. U.S. Pat. No. 5,514,375 discloses various recombinant vaccinia viruses which express portions of the JEV open reading frame extending from prM to NS2B. These pox viruses induced formation of extracellular particles that contain the processed M protein and the E protein. Two recombinant viruses encoding these JEV proteins produced high titers of neutralizing and hemagglutinin-inhibiting antibodies, and protective immunity, in mice. The extent of these effects was greater after two immunization treatments than after only one. Recombinant vaccinia virus containing genes for the prM/M and E proteins of JEV conferred protective immunity when administered to mice (Konishi et al., Virology 180: 401-410 (1991)). HeLa cells infected with recombinant vaccinia virus bearing genes for prM and E from JEV were shown to produce subviral particles (Konishi et al., Virology 188: 714-720 (1992)). Dmitriev et al. reported immunization of mice with a recombinant vaccinia virus encoding structural and certain nonstructural proteins from tick-borne encephalitis virus (J. Biotechnology 44: 97-103 (1996)).
Recombinant virus vectors have also been prepared to serve as virus vaccines for dengue fever. Zhao et al. (J. Virol. 61: 4019-4022 (1987)) prepared recombinant vaccinia virus bearing structural proteins and NS1 from dengue serotype 4 and achieved expression after infecting mammalian cells with the recombinant virus. Similar expression was obtained using recombinant baculovirus to infect target insect cells (Zhang et al., J. Virol. 62: 3027-3031(1988)). Bray et al. (J. Virol. 63: 2853-2856 (1989)) also reported a recombinant vaccinia dengue vaccine based on the E protein gene that confers protective immunity to mice against dengue encephalitis when challenged. Falgout et al. (J. Virol 63: 1852-1860 (1989)) and Falgout et al. (J. Virol. 64: 4356-4363 (1990)) reported similar results. Zhang et al. (J. Virol 62: 3027-3031 (1988)) showed that recombinant baculovirus encoding dengue E and NS1 proteins likewise protected mice against dengue encephalitis when challenged. Other combinations in which structural and nonstructural genes were incorporated into recombinant virus vaccines failed to produce significant immunity (Bray et al., J. Virol. 63: 2853-2856 (1989)). Also, monkeys failed to develop fully protective immunity to dengue virus challenge when immunized with recombinant baculovirus expressing the E protein (Lai et al. (1990) pp. 119-124 in F. Brown, R. M. Chancock, H. S. Ginsberg and R. Lerner (eds.) Vaccines 90: Modern approaches to new vaccines including prevention of AIDS, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.).
Immunization using recombinant DNA preparations has been reported for SLEV and dengue-2 virus, using weanling mice as the model (Phillpotts et al., Arch. Virol. 141: 743-749 (1996); Kochel et al., Vaccine 15: 547-552 (1997)). Plasmid DNA encoding the prM and E genes of SLEV provided partial protection against SLEV challenge with a single or double dose of DNA immunization. In these experiments, control mice exhibited about 25% survival and no protective antibody was detected in the DNA-immunized mice (Phillpotts et al., Arch. Virol. 141: 743-749 (1996)). In mice that received three intradermal injections of recombinant dengue-2 plasmid DNA containing prM, 100% developed anti-dengue-2 neutralizing antibodies and 92% of those receiving the corresponding E gene likewise developed neutralizing antibodies (Kochel et al., Vaccine 15: 547-552 (1997)). Challenge experiments using a two-dose schedule, however, failed to protect mice against lethal dengue-2 virus challenge.
The vaccines developed to date for immunizing against infection by JEV, SLEV, dengue virus and other flaviviruses have a number of disadvantages and problems attending their use. Inactivated vaccine is costly and inconvenient to prepare. In addition, any such vaccine entails the risk of allergic reaction originating from proteins of the host cell used in preparing the virus. Furthermore, such vaccines present considerable risk to the workers employed in their production. Candidate attenuated JEV vaccines are undergoing clinical trials, but as of 1996 have not found wide acceptance outside of the People's Republic of China (Hennessy et al., Lancet 347: 1583-1586 (1996)).
Recombinant vaccines based on the use of only certain proteins of flaviviruses, such as JEV, produced by biosynthetic expression in cell culture with subsequent purification or treatment of antigens, do not induce high antibody titers. Also, like the whole virus preparations, these vaccines carry the risk of adverse allergic reaction to antigens from the host or to the vector. Vaccine development against dengue virus and WNV is less advanced and such virus-based or recombinant protein-based vaccines face problems similar to those alluded to above.
There is therefore a need for vaccines or improved vaccines directed against flaviviruses such as yellow fever virus, dengue virus, JEV, SLEV and WNV which are inexpensive to prepare, present little risk to workers involved in their manufacture, carry minimal risk of adverse immunological reactions due to impurities or adventitious immunogenic components and are highly effective in eliciting neutralizing antibodies and protective immunity. There is furthermore a need for a vaccine against JEV, WNV and related flaviviruses that minimizes the number of immunizing doses required.
Many of the shortcomings of the current art as described in detail for the production of vaccines also apply to the production of antigens and antibodies to be used for the production of immunodiagnostics. Particularly, the concurrent risks and costs involved in the production of antigens from viruses and the failure of most currently available recombinantly expressed antigens to elicit effective immune responses are paralleled in the field of immunodiagnostics by the same risks, high costs and a corresponding lack of sensitivity. Thus, because of the high costs, risk of accidental infection with live virus and the lower than desired levels of sensitivity of the previously available tests, there exists a need for rapid, simple and highly sensitive diagnostic tests for detecting flavivirus infection and/or contamination.
The present invention meets these needs by providing highly immunogenic recombinant antigens for use in diagnostic assays for the detection of antibodies to selected flaviviruses. The present invention further provides for the use of recombinant antigens derived from flaviviruses, flavivirus genes or mimetics thereof in immunodiagnostic assays for the detection of antibodies to flavivirus proteins.